Optical data readers include devices for sensing illumination and typically illumination that is reflected by a surface being read. One type of data reader is an optical scanner for reading optical codes comprised of dark elements separated by white or light-colored spaces. A 1-D barcode (such as a UPC or EAN/JAN barcode) is an example of an optical code readable by such an optical scanner, although 2-D codes, such as PDF-417 and Maxicode, are also readable using similar methods and equipment. Other types of optical data readers are useful for reading other image data and other kinds of symbols.
Imaging devices such as charge-coupled devices (CCDs) and complementary metal oxide semiconductor (CMOS) imagers can be used to capture image data for use in data reading applications. In such devices, an image of the optical code or other scene is focused onto a detector array. With some such devices, it may be advantageous to provide a source of illumination that illuminates the optical code or other scene being imaged, to provide the required signal response in the imaging device. Such a source of illumination can reduce exposure time, thereby improving imager performance, especially in low ambient light conditions and when imaging moving items.
U.S. Provisional Patent Application No. 60/632,696 (“the '696 application”) describes some potential advantages of utilizing pulsed illumination generated by an LED or other illumination source. The '696 application also describes the use of multiple different wavelengths of illumination in connection with a frame-shuttered imaging data reader, which may be useful for reading a variety of optical codes having different surface reflection characteristics. For example, infrared illumination in the range of 850 nm may result in improved imaging performance for many optical code surfaces, and infrared illumination has the added benefit of being invisible to the human eye. On the other hand, thermally printed barcodes have little or no contrast in the infrared spectrum, but good contrast in the visible spectrum at around 590 nm. In some instances, a single optical code may be more easily read by utilizing illumination of multiple different wavelengths. Thus, the present inventor has recognized a need for providing multiple wavelengths of illumination for a data reader and has proposed alternating between different illumination wavelengths for consecutive or sequential exposures of the imager to avoid interference and improve imager performance. Since infrared illumination is not visible, it can be pulsed a high power only when needed, during the exposure interval of selected frames of the imager. Visible illumination, however, must be pulsed at a lower luminous power and at a rate that is higher than the flicker fusion frequency of human vision (typically about 60 Hz) to avoid distracting users and bystanders. Unfortunately, imager frame rates may need to be slower (less) than the flicker fusion frequency, due to design constraints of the imager or other reasons; and, in such cases, the visible illumination cannot merely be triggered to coincide with the exposure intervals of the imager without resulting in a flicker effect. And even if the imager frame rate is greater than the flicker fusion frequency, if the visible illumination is not pulsed during every frame exposure (for example when multiple light sources of different wavelengths are used), then the resulting visible illumination pulse frequency may be lower than the flicker fusion frequency.
The present inventor has, therefore, recognized a need for improved methods of pulsing illumination for a data reader. The present inventor has also recognized a need for methods of pulsing illumination in conjunction with operating an imager; and, in particular, such methods that avoid the flicker effect while ensuring that illumination is pulsed during an imager's frame exposures or some subset thereof. The present inventor has also recognized a need for illumination pulsing methods that facilitate illumination with different wavelengths during sequential frame exposures of an imager.